Performance curve conversion for impeller trimming

Impeller trimming is the reduction of the impeller diameter of a centrifugal pump with the aim of adapting the operating point to the specifications. The following applies approximately: Q = flow rate
D = impeller diameter
r = index for the reduced impeller diameter
t = index for the reference wheel diameter

The throttle curve H (Q) can be roughly determined from this relationship.

A more precise calculation, however, requires the consideration of performance charts in which an impeller diameter is assigned to each performance curve. The new performance curve is determined by interpolating the conversion from the neighboring curves. In order to fully utilize the efficiency of the method, it is recommended to record an duty chart with at least three performance curves. If there is a large difference between the smallest and largest impeller diameter, some (2..4) intermediate curves are required.

An alternative calculation method is described in ISO 9906. Knowledge of the mean impeller diameter at the leading edge D 1 is required here. According to the standard, this procedure is valid for

• Diameter reduction up to max. 5%
• Type number K ≤ 1.5
• Unchanged blade geometry (outlet angle, tapering, etc.) after cutting D 1 = Mean diameter at the impeller leading edge

For pumps with a type number K ≤ 1.0 and a maximum impeller diameter reduction of 3%, the efficiency can be considered as unaltered.

More articles on this topic

Speed – Affinity Laws

The affinity law describes the speed dependency of the delivery parameters of centrifugal pumps. It can be derived from the general similarity relationships as a special case.

Pump Performance Chart

The pump performance chart is a summarized diagram containing multiple pump performance curves.

Duty Point

The duty point of the system is the required operating point determined by means of a design calculation (theoretical value).

Impeller

The impeller is the rotating, bladed component of a centrifugal pump through which the flow diversion is caused.